He arrived laughing and joking. Just over two hours later Armin Meiwes, the self-confessed German cannibal who killed and ate another man, left a stunned courtroom scarcely able to believe his luck.

After a unique two-month trial that has gripped both Germany and the wider world with its gory details, a judge yesterday sentenced Meiwes to a mere eight and a half years in jail after ruling that he was guilty of manslaughter and not murder.

The prosecution had demanded a 15-year-term. But Judge Volker Mütze said he accepted the cannibal's argument that the man he met through the internet and later fried in garlic - 43-year-old computer engineer Bernd Jürgen Brandes - had wanted to die.

"The famous lust for murder was not there. The killing was very unpleasant for Meiwes," the judge said. "There were no base motives." He added: "These were two psychologically sick people who found each other. A door has been opened into a world which we immediately want to shut, a grey zone where people gather on the internet to live out their fantasies."

German prosecutors yesterday said they were considering whether to appeal against the verdict. "We believe this was murder," prosecutor Andreas Köhler said.

The cannibal's lawyer, Harald Ermel, said his client was "satisfied" with the result, and was now considering numerous offers from film companies and publishers. With good behaviour, Meiwes was likely to be free in four and a half years, Mr Ermel pointed out. He would probably write his memoirs.

There was no reason why he should not eventually return home to the timbered farmhouse in Rotenburg, central Germany, where he cut up and refrigerated bits of Brandes, burying his skull in the garden, the lawyer said. "He isn't a murderer," he added. "He has to live."

Justifying his verdict, Judge Mütze yesterday described how 42-year-old Meiwes had experienced cannibal feelings as a young boy, after his father and two brothers walked out on the family home. He had also fantasised about eating Sandy, the blond boy hero of the Sixties TV series Flipper.

These feelings got stronger after the death of his "domineering" mother in 1999 and eventually led him to place an advert on the internet seeking someone to "slaughter and eat", the court heard. He got a reply, from Bernd Brandes, an outwardly successful manager with Siemens in Berlin with secret masochistic fantasies.

"The victim offered up his body because he wanted to get the kick of his life," the judge told the court in Kassel, as Meiwes sitting next to his lawyer listened avidly. It was, the judge said, "to be the final act, the high point of his life".

In an email read to the court yesterday Brandes told the cannibal: "I hope you are serious because I really want it. My nipples look forward to your stomach ..."

In March 2001 Brandes took the day off work, wiped all evidence of his masochistic fantasies from his computer and travelled to Rotenburg by train. He met Meiwes and went to the isolated farmhouse where he lived alone. Meiwes then showed Brandes the "slaughter room" he had constructed upstairs.

Later that evening Meiwes cut off Brandes' penis and both men tried - and failed - to eat it. Meiwes then ran a bath for Brandes, the judge said, who by this stage had knocked back 20 sleeping tablets and a bottle of Vick's cold cure. Meiwes waited for his victim to pass out by reading a Star Trek novel. At 3.30am Meiwes laid his unconscious victim on the slaughter table and stabbed him to death. He then hung him on a hook, cut out some of his organs, and went to bed.

The court in Kassel was told that the cannibal videoed the entire episode, and had told the dying Brandes: "I can't do anything else." The next afternoon he set about cutting off parts of Brandes, which he stored in his freezer.

By the time detectives finally knocked on the door of his farmhouse in December 2002, he had eaten "about 20kg" of his victim, the judge said.

In an interview with Stern magazine shortly before his trial, the cannibal admitted to cooking parts of Brandes with olive oil, garlic, pepper and nutmeg, and eating him with sprouts, potatoes and a bottle of South African red wine on a festively decorated table.

Brandes also told the magazine he had used one of his victim's feet as a table ornament and had tried to make flour by grating an arm bone after first baking it in the oven. He buried Brandes' bones, skin and innards in the garden while reciting the 23rd psalm: "The Lord is my shepherd and I shall not want ..."

The magazine also revealed that Meiwes believed that after eating Brandes, who spoke good English, his own English had improved.

Yesterday Judge Mütze admitted that the cannibal's behaviour clearly breached what he called "civilisational norms". He also accepted the testimony of expert witnesses who had earlier told the court Meiwes was mentally capable of standing trial.

But there was little evidence to support the prosecution's main contention that this was a sexual murder, carried out for the purposes of gratification, he ruled. After the killing, Meiwes had watched his homemade video "two or three times" while sitting in his armchair, the judge said, but it was not clear whether he was sexually aroused. "There was an agreement between them. This was the killing of a person without murder," he concluded.

Last night legal experts described the case and its extraordinary denouement as unprecedented. "This will make legal history," Professor Arthur Kreuzer of the Institute for Criminology at Giessen University said. "The killer sought out his victim and the victim sought out his killer."

Police revealed during the trial that there were at least 800 participants in cannibal forums on the internet and two suspected cases of other cannibal killings. Detectives said Meiwes had been questioned as part of investigations in Frankfurt and in Austria. He has not been charged with these murders.

After dismembering Brandes, Miewes returned to the internet and placed adverts asking for further volunteers for slaughter. He met five other people, including a German hotel manager working in London, but did not kill any of them after they failed to give him their consent.

Several of the witnesses gave anonymous evidence in court which appeared to back Meiwes' assertion that he was only interested in willing victims. One of them - a German cook - turned up wearing ski goggles to avoid being identified.

The cannibal's defence team had claimed that he was guily of "killing on demand" - a lesser charge with a maximum five-year sentence.

Last night Meiwes' lawyer said it was unlikely his client would eat anyone else. "He certainly won't repeat it," he said. Miewes had been in jail for a year. He was now a polite, popular prisoner who helped others and wrote their letters, he added.